Field of the Invention
This invention pertains to video signal processing circuits and, in particular, to video signal processing circuits that remove a synchronization pulse from a video signal.
Certain video processing applications require the removal of synchronization pulses in order to operate correctly. For example, a thermal printer responsive to a video input ordinarily processes color information without the presence of synchronization information. The typical video input available to a thermal printer, however, includes synchronization information, e.g., a video signal with a positive-going voltage for white and a negative-going voltage for the synchronization pulse (hereinafter referred to as the sync pulse or the sync component). A setup voltage immediately follows the sync pulse and closely approximates the black voltage of the video signal. (For purposes of this patent application, "black voltage" will therefore refer to the setup voltage, and will be used interchangeably therewith.) A sync removal circuit removes the sync pulse from the video signal and replaces it with a voltage representing black.
FIG. 1 shows a known circuit for removing the sync pulse from a video signal. Pertinent waveforms are shown in FIG. 2. An input video signal A is ac-coupled through a capacitor 10 to a bipolar transistor 12 arranged as an emitter follower. A clamping section includes a gating transistor 14 having its collector connected to junction 16 and its emitter connected to a transistor 18 arranged to function as a diode. The transistor 18 is forward-biased by application of a voltage B+ through a resistor 20. When a black clamp signal C is applied to the base of the transistor 14, the pulses thereof turn on the transistor 14 and clamp the corresponding portions of the ac-coupled video signal A to the collector voltage V (0.7 volts) of the forward-biased transistor 18. The resultant signal at the junction 16 is illustrated by the video waveform B. Consequently, the transistor 12 passes the portions of the video signal greater than the voltage V, that is, the portions that are positive-going from black. However, the sync pulse in lost in the video output signal D since the transistor 12 is effectively biased to cut off at the clamping level, i.e., the base-to-emitter turn-on voltage of the transistor 12.
This known circuit has several problems. Precise removal of the sync pulse depends on close matching of the voltage drop of the base-to-emitter junctions of the bipolar transistors 12 and 18. Any disparity in these voltages will either leave part of the sync pulse or cut off part of the black information. Furthermore, video levels slightly positive to black will be nonlinear because they are close to the cutoff voltage of the bipolar transistor 12.